In the preparation of phosphoric acid finely ground phosphate rock is slurried with water, recycled phosphoric acid slurry, and sulfuric acid and heated at 165-190.degree. F. for 2-8 hours. This allows the conversion of the phosphate component of the phosphate rock to phosphoric acid and the precipitation of calcium component of the phosphate rock as gypsum many times in the form of calcium sulfate dihydrate. The undissolved rock and the newly formed gypsum together with gangue are removed from the phosphoric acid liquid phase by a filtration step. The phosphoric acid industry uses horizontal pan filters almost exclusively in this process (usually Prayon Filters manufactured by Bird Engineering). The filtrate typically ( 26-32% P.sub.2 O.sub.5 or 40% H.sub.3 PO.sub.4) is retained and the filter cake is discharged as waste. The acid solution is typically concentrated in vacuum evaporators to 54% P.sub.2 O.sub.5 ( 100% H.sub.3 PO.sub.4) or concentrated further in super acid evaporates to as high as 72% P.sub.2 O.sub.5.
If phosphoric acid production in the plant is limited by filter capacity, improvements in filter efficiency can have a large commercial impact. In general, a demonstrated improvement in filtration rates exceeding 5% is considered very significant.
The conditioning of filter feed with high molecular weight polyacrylamide and acrylamide/sodium acrylate copolymer to improve gypsum filtration rates has been previously reported in the literature. However, no reference teaches the use of sulfonated acrylamide copolymers to improve gypsum filtration rates.
In fact, U.S. Pat. No. 3,644,091 teaches that sulfonated polyacrylamide and its sodium salts are noneffective in coagulating dispersed solids in the wet phosphoric acid process.
It would be beneficial to find other polymers which would be useful as filtration aids to improve the filtration rates in the production of phosphoric acid.